\[\boxed{\text{847\ (847).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[Пусть\ в\ группе\ было\ \text{x\ }\]
\[туристов:\]
\[C_{x}^{2} = \frac{x!}{(x - 2)!} =\]
\[= \frac{(x - 2)!(x - 1) \cdot x}{(x - 2)!} =\]
\[= (x - 1) \cdot x.\]
\[Если\ бы\ было\ на\ одного\ \]
\[человека\ больше,\ то:\]
\[C_{x + 1}^{2} = \frac{(x + 1)!}{\left( (x + 1) - 2 \right)!} =\]
\[= \frac{(x + 1)!}{(x - 1)!} =\]
\[= \frac{(x - 1)!\left( x \cdot (x + 1) \right)!}{(x - 1)!} =\]
\[= x(x + 1).\]
\[\frac{A_{x + 1}^{2}}{A_{x}^{2}} = 1,25\]
\[\frac{(x + 1)x}{x(x - 1)} = 1\frac{1}{4}\]
\[\frac{x + 1}{x - 1} = \frac{5}{4}\]
\[4 \cdot (x + 1) = 5 \cdot (x - 1)\]
\[4x + 4 = 5x - 5\]
\[- x = - 9\]
\[x = 9\]
\[Ответ:9\ туристов.\]